Failsafe wheel slip control system and method of operating same

ABSTRACT

A slip control system for vehicles having spaced apart drive wheels (200,202) driven by an input shaft (204) through a differential unit (206) including means (220) for calculating wheel slip according to the ratio of wheel speeds, means (220,230,232) for entering a slip control mode wherein a braking force is applied to the slipping wheel and incrementally varied on a periodic review basis according to recalculated slip values. If slip increases or holds steady, the brake force is incrementally increased. As slip becomes less, the brake force is incrementally reduced, the particular increments of force reduction being selected in accordance with successive measurements of slip. A drive shaft signal (226) is compared to the high wheel speed signal to identify a wheel speed transducer failure.

TECHNICAL FIELD

This invention relates to slip control systems for vehicles havingdifferentially driven road wheels in which slip is controlled byapplication of a braking force to the spinning wheel, and moreparticularly, to a method and apparatus for detecting a wheel speedtransducer failure by comparing the spinning wheel speed to the driveshaft speed.

BACKGROUND ART

It is well known that the standard vehicle having spaced apart drivewheels or wheel sets which are powered by a single engine through adifferential drive experiences difficulties when one of the twodifferentially driven wheels loses traction. Conditions which give riseto a loss of traction exist commonly in construction sites and otheroff-road locations as well as on normal roads during wet, snowy or icyweather. A truck or automobile having one of two differentially drivenwheels or wheel sets on ice and the other on ground providing goodtraction is often unable to move due to the fact that the action of thedifferential drive system directs all power to the wheel having notraction. The result is a slip condition wherein the wheel withouttraction rotates at twice its normal speed under given gear ratiospecifications and the wheel with traction remains stationary.

To alleviate the slip or loss of traction condition, various mechanicalanti-spin devices have been developed and put into commercial use. Suchdevices, however, can produce an abrupt transfer of all driving power tothe wheel or wheel set having traction. This abrupt and full powertransfer can create such mechanical stresses as to shorten the usefullife of the drive train and/or cause catastrophic failure. In addition,mechanical anti-spin units often fail to accommodate the wheel speeddifferential which arises during normal turning of the vehicle and hencegive rise to excessive tire wear due to drag effect.

An alternative approach to the slip problem due to loss of traction indifferentially driven vehicles involves the provision of separatelyactuable drive wheel brakes whereby the operator can selectively apply abraking force to the spinning or slipping wheel thus to effect abalancing of power as between the slipping and nonslipping wheels; i.e.,the application of the braking force to the slipping wheel simulatesincreased rolling resistance and results in a more even distribution ofpower as between the two differentially driven wheels. Systems of thistype are common on farm vehicles but are not believed to be practical onlarge transport or off-road vehicles such as trucks and road graders.

More sophisticated approaches to slip control using the selectivelyactuable wheel brake systems are known in the prior art. These systemsinclude speed sensors disposed on or adjacent each of the differentiallydriven wheels for generating speed signals, means for comparing the twosignals to develop a slip signal and selectively operated solenoid meansor solenoid operated valves to actuate either the left or right wheelbrake when a slip condition is detected. One such system is disclosed inthe U.S. Pat. No. 4,066,300 to Devlin issued Jan. 3, 1978. Another suchsystem is disclosed in the U.S. Pat. No. 3,025,772 to Eger, Jr. et al.issued Mar. 20, 1962.

Neither the Devlin nor Eger, Jr. et al. system provides effective meansfor distinguishing between the signal resulting from a transducerfailure and the signal resulting from a true slip condition wherein onewheel has lost traction. Both conditions include an indication that onewheel is stationary and the other is turning. Accordingly, automaticsystem actuation may occur when it is neither required nor advantageousto vehicle operation.

Another related prior art system is disclosed in the U.S. Pat. No.3,871,249 to Jeffers, issued Mar. 18, 1975. In the Jeffers system, amechanical locking device for a differential is equipped with a driveshaft speed sensor which inhibits lockup if shaft speed exceeds somepredetermined amount. No means are provided for determining therelationship between shaft and wheel speeds. Hence Jeffers is notconcerned with the problem of transducer failure and gives no teachingwith respect to it.

The present invention is directed to overcoming the problems of theprior art and to provide an improved vehicle slip control system whereinslip is positively distinguished from transducer failure.

DISCLOSURE OF THE INVENTION

In one aspect of the invention an apparatus is provided forautomatically balancing the power transfer between two differentiallydriven vehicle wheels when one of the wheels loses traction by applyinga proportionally varying braking force to the wheel which loses tractionduring a slip control time period. Moreover, the system includes meanswhereby a true slip condition is positively distinguished from a brokenor failed wheel speed transducer condition. This system comprises knownwheel speed transducers for generating wheel speed signals and, inaddition, a third transducer is associated with the drive shaft, or acomponent rotating in unison therewith, to generate an input speedsignal. Means are further provided for comparing the input speed withthe spinning wheel speed and for entering a slip control mode only ifthe ratio of wheel to input speeds exceeds a predetermined value.

In another aspect of the invention, a method is provided for achievingimproved operation of an anti-slip system for vehicles having wheels orwheel sets driven through a differential unit by an input shaft whereina failure of one wheel speed transducer is positively distinguished froma true slip condition. This is achieved by comparing the speed of thespinning wheel to the speed of the input shaft and entering a slipcontrol mode only when the speed ratio exceeds a predetermined value.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a vehicle drive system embodying ananti-slip control system in accordance with the invention;

FIG. 2 is a block diagram of a wheel slip control system in accordancewith the invention and comprising a microprocessor to performmathematical calculations and comparisons;

FIG. 3 is a more detailed block diagram of a portion of the overallsystem of FIG. 2;

FIG. 4 which comprises FIGS. 4A and 4B is an operational flow chart fromwhich programming for the microprocessor in the embodiment of FIG. 2 maybe readily developed, and

FIG. 5 is a chart of a brake hold off pressure versus time and isutilized in describing the operation of the embodiment of FIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 illustrates an embodiment of the invention as applied to thedetection and control of slip due to loss of traction in a vehiclehaving road wheels 200 and 202 driven by a single engine (not shown)through an input or drive shaft 204, a differential unit 206 andhalf-axles 208 and 210 respectively. The drive system is per seconventional and no details beyond these need be given for anunderstanding of the invention.

Wheels 200, 202 are stopped by spring engaged emergency/parking brakepistons or hydraulically engaged service brake pistons of brakes 212,214. The brakes are spring biased so that when no hydraulic pressure ispresent, the emergency/parking brakes engage and stop the wheels. Duringoperation of the vehicle the brakes are maintained in the disengagedposition by fluid pressure as disclosed in U.S. Pat. No. 3,927,737issued Dec. 23, 1975 to P. F. M. Prillinger and assigned to the assigneeof this invention. The brakes are normally actuated via a service brakeline 237 coming from the rear wheel service brake and retarder mastercylinders (now shown). The service braking system, per se, is well knownand does not form part of this invention. The brakes are also actuatedthrough the parking/emergency brake lines 236, 238 as described indetail below.

The slip control system comprises a left wheel speed pickup in the formof an electromagnetic transducer 216 which operates to provide pulses incooperation with a gear-like device 218 which is mounted in the housingfor axle 208 so as to be rotated with said axle. Signals from pickup 216are applied to one input of an electronic controller 220, the details ofwhich are hereinafter described. The right wheel speed signals providedby means of a pickup 222 operate in conjunction with a gear-like device224 which rotates with the axle 210. The right wheel speed signal isapplied to another input of electronic controller 220. Finally, a driveshaft speed signal is generated by pickup 226 and gear-like device 228which rotates with the drive shaft 204. The drive shaft speed signalsare applied to a third input of electronic controller 220.

All of the transducers 216, 222 and 226 are preferably electromagnetictype devices for measuring a change in magnetic reluctance due to avarying air gap between the pickup pole and a rotating ferrous disk orgear. Such devices produce a time varying output voltage which is easilyamplified and shaped into square pulses or spikes in a known manner.However, other known transducers, such as optical and hall effectdevices may be employed as alternatives.

Controller 220 operates upon the three signal inputs to determine theexistence, magnitude and location of wheel slip during a loss oftraction situation, and to distinguish between wheel slip and atransducer failure. If a true slip condition is indicated, the powertransfer between the two differentially driven vehicle wheels 200 and202 is balanced by applying a proportional braking force to the wheelwhich loses traction. This is accomplished by means of locationselection valve 230 and proportioning valve 232, both of which areconnected to receive output signals from the controller 220. The valves230 and 232 operate in combination with a supply 234 of oil or brakefluid under pressure, the fluid lines from supply 234 running boththrough the proportioning valve 232 and around the proportioning valveto the 4-way solenoid operated valve 230 which directs full pressure toone of the parking brakes 212,214, and modulated or proportionallycontrolled fluid pressure to the other. In this instance brake pressureis applied by relieving fluid pressure in one or the other of the twobrake lines 236 and 238. This is a consequence of the selection ofspring biased brakes 212, 214 and could be straightforwardly implementedin the reverse fashion, i.e., brake pressure might be increased indirect rather than inverse ratio to the applied fluid pressure.

FIG. 2 illustrates the preferred implementation of the electroniccontroller 220 of FIG. 1. In this implementation a solid state digitalmicroprocessor 242 of the type available from such sources as Motorolaand Fairchild is utilized to perform system control functions; mostnotably, to compare the speed of the spinning wheel to the speed of theinput shaft 204 and to enter the slip control made only if the speedratio matches or exceeds a given value. Microprocessor 242 also isprogrammed to establish a plurality of slip value bands, each bandhaving defined slip signal values and each band having associatedtherewith an applied brake force value expressed in terms of fluidpressure. The microprocessor is readily programmed to respond to signalsfrom the left, right and drive shaft pickups 216, 222 and 226,respectively, to generate a slip signal and to compare the slip signalto the limits of each of the bands, thereby to determine the particularband within which the slip signal resides in any given time. To this endmicroprocessor 242 is operatively interconnected with a band valuememory 244 and a timer 246 which establishes a timing cycle hereinafterdescribed. Pickups 216,222 and 226 are connected through an inputprotection circuit and noise filter circuit which provides appropriatelydigitized input signals to the microprocessor 242. A retard brakepressure switch 248 and a service brake pressure switch 250 areconnected through input protection and noise filter circuit block 252 tothe microprocessor 242 for resetting the anti-spin control to OFF whenthe service brakes or retarder are actuated, much the same as anautomobile cruise control is reset OFF when the service brakes areapplied. This feature gives the vehicle operator uncontested control ofthe braking function when he is using the service brakes.

Completing the description of the block diagram of the system as shownin FIG. 2, microprocessor 242 has three illustrated outputs, the firstoutput going through a pulse width modulated servo valve driver 254 tothe servo operated proportioning valve 232. The second output goes tothe solenoid driver 234 associated with the left direction shuttle valve230a and the third output goes through solenoid driver 236 to the rightdirection shuttle valve 230b. It will be appreciated that the outputs tothe solenoids 230a and 230b are represented in FIG. 1 as a single linefor purposes of simplicity.

FIG. 3 illustrates a preferred implementation of the pulse widthmodulated servo valve driver 254 in greater detail. The pulse widthmodulation servo valve driver 254 comprises a duty cycle generator 256in the form of a digital to pulse width converter, such as Motorolaintegrated circuit number 6840. An oscillator 258 is connected to theduty cycle generator 256 to provide a base frequency. The output of theduty cycle generator is a width modulated pulse train at a frequency of100 Hz and this signal is applied to an amplifier 260 which increasesthe amplitude of the signal as shown. The signal is applied to aproportioning valve 262, such as the proportional controller modelnumber 82 which is available from the Fema Corporation of Portage, Mich.The proportional controller is in turn connected to the control valve232 in a manner well known to those skilled in the art to vent thepressure from supply 234 and thereby modulate the brake fluid pressurein whichever of the lines 236 and 238 is selected by valve 230.

Referring now to FIG. 4, a flow chart defining the internal programmingfor the microprocessor 242 and associated memory 244 is shown. From thisflow chart a programmer of ordinary skill can develop a specific set ofinstructions for a general purpose microprocessor so as to define thenecessary slip signal value bands, timing cycles, and brake fluidpressure values which are essential to the full implementation of theinvention as hereinafter described. It will be appreciated that whilethe best mode of the invention comprises the properly programmedmicroprocessor, the programming of which is disclosed in FIG. 4 and theresult of which is the creation of novel hardware associations withinthe microprocessor and its associated devices, it is possible toimplement the invention utilizing more traditional hardwired circuitsincluding comparators, summers and gates. For example, slip signalvalues may be generated by means of tachometer generators and voltagecomparators and the result applied to a series of biased comparatoramplifiers representing a series of contiguous slip value bands. Theoutputs of the amplifiers may be summed through conventional logic andapplied to a stepper motor, ladder network or other summing deviceoperatively interconnected with the proportioning valve.

INDUSTRIAL APPLICABILITY

Referring now to FIGS. 1, 4 and 5, an example of industrialapplicability will be described with reference to the operation of avehicle having differentially driven wheels 200 and 202, a drive shaft204 and a differential unit 206 through which the wheels 200 and 202receive power from a single engine. It is further assumed that thevehicle on which the wheels 200 and 202 are mounted has a turningcapability which, when employed to full effect, results in a maximumwheel speed differential of 1.5.

By way of preliminary summary, the operation of the embodiment of FIGS.1 and 2, when programmed in accordance with the flow chart of FIG. 4,involves the definition and storage of five speed differential or slipbands and the further definition of braking forces, expressed in brakefluid pressure increments, associated with a speed differential valuefor each band. In addition, a relatively high slip value threshold isset for entering into the slip control mode while a relatively low slipthreshold value is established for exiting from the slip control mode.In addition, a timing period is established whereby the slip value isperiodically redetermined and, to the extent it remains within a givenband, a brake pressure increment, either positive or negative, iseffected at the end of each clock time until the slip value moves toanother band. Finally, an indication of slip is always cross-checked bycomparing the speed of the spinning wheel to the speed of the driveshaft 204 so that the functional failure of one wheel speed pickup isnot misinterpreted as a slip condition. This latter function derivesfrom the phenomenon of differential drives wherein a 100% slip conditionresults in a ratio of slipping wheel speed to drive shaft speed which isdouble the normal ratio existing under non-slip, non-turning conditions.Assuming a 1:1 rear axle ratio, this can be simplified by saying that a100% slip condition exists when the slipping wheel speed is twice thedrive shaft speed.

Having briefly summarized the functional characteristics of the properlyprogrammed system a full description of operation will now be given.

Flow chart block 264 (FIG. 4) represents the sampling of data from theleft and right speed signal pickups 216 and 222; it will be appreciatedthat such data is placed in given address locations by themicroprocessor so that it is available for later retrieval andprocessing for calculation purposes. As a first condition it is assumedthat one of the axle speed signals is equal to zero. If this is the casethe routine progresses along the right side of the flow diagram in FIG.4. Block 266 represents input of the drive shaft speed signal frompickup 226, it being understood that this signal is also placed in apredetermined storage location by the microprocessor. The input or driveshaft speed signal is compared in block 268 with the non-zero wheelspeed signal and if the ratio of the wheel speed to drive shaft speed isequal to or less than 1.5 it is assumed that the wheel showing zerospeed is actually turning and the zero indication is the result of apickup failure. The program then calls for a return to the originalinput point; in addition it may be desirable to set an output signalcondition which alerts the operator to the apparent transducer failure.

If the rotating wheel speed is greater than 1.5 times the drive shaftspeed, as indicated by comparison steps 268, the program progresses tothe evaluative step represented by flow chart blocks 270a, 270b and270c. If the condition represented by block 270a is satisfied theprogram progresses to block 272a which is an event counter requiringseveral successive cycles of positive signal conditions before the"brake latch" condition, i.e., the activation of the slip control systemis begun. It has been found that such a delay is advisable from thestandpoint of efficient system operation so as to filter out short termaberrations in wheel speed condition. If the event counter has notreached the delay time α the program progresses to block 274a whichinvolves incrementing the counter. From this point the program returnsto the original input condition.

If the condition represented by program block 270b is satisfied theprogram progresses to blocks 272b and 274b in the manner previouslydescribed. If either block 272a or 272b produces a positive signalcondition the program progresses to block 276 which involves adetermination of the input signal switch conditions controlling theservice brake, the retarder and/or the vehicle speed. In addition, itmay be advisable to check brake pressure.

If the speed signal condition represented by block 270c is satisfied theprogram advances to blocks 278, 280, 282 and 284 to release the parkingbrakes 212 and 214 and reset the timer. The same result occurs if theoutput of program block 276 is positive.

If the output of program 276 is negative the program progresses to block286 to determine whether the slip control system is activated. If theresult is positive the effect is the same as entering band 5, i.e., themost severe slip condition band and progressing to block 294 whichincreases the wheel brake force by decreasing the wheel brake hold-offpressure by the pressure increment x for every timed intervalestablished by the microprocessor timer. If program block 286 indicatesthat the slip control system is not operative the program progresses toblock 288 which activates the system and thence through blocks 290 and292 to begin the timing cycle and reduce the brake hold off pressure toa value just sufficient to maintain the brakes in the off position. Onthe next pass through the above sequence, a positive result will beobtained at block 286 and in block 294 brake hold off pressure will bedecremented x psi. The control will continue to repeat this sequence attimed intervals until either the zero speed wheel starts to turn or elsethe brake hold off pressure is decremented to nearly zero psi (<10 psi).In the latter case, the brakes are then released completely. In theformer case, the control follows the path from block 264 to block 296and beyond as described in the following paragraphs.

Assuming the result of calculation block 264 indicates that neitherwheels 200 nor 202 is stopped; i.e., the vehicle is moving, the programbranches to block 296 representing a calculation subroutine. In essence,the calculation subroutine involves determining the location of theactual slip value within each of five bands represented by the smalltable shown to the upper left side of FIG. 4. Note that the slip bandsare contiguous, i.e., the upper limit of one band is the lower limit ofthe next band and that the highest slip value which is required foractivation is 1.7.

Program blocks 298, 300 and 302 represent the determinations of whichwheel is the faster rotating wheel and the proper disposition of theshuttle valve 230 so as to direct modulated brake pressure to thehighest speed wheel and unmodulated brake pressure to the lower speedwheel. The program then advances to block 304 to determine whether ornot a sufficient slip time period has elapsed to begin operation of thesystem. If insufficient time has elapsed program block 306 simplyincrements the counter and begins the process over again. If sufficienttime has elapsed the program advances to block 308 which scans thevarious signal conditions which might disqualify the vehicle foroperation of the slip control system. If, however, the vehicle qualifiesfor slip control the program advances to a selected one of the bands310, 312, 314, 316 and 318. Assuming that a slip condition is firstdetermined and that the time α has elapsed, the vehicle can only enterthe slip control mode via band 5; the ratio of the high speed wheel rateto the low speed wheel rate must be 1.7 or greater. However, ashereinafter described, the vehicle can exit the control mode bysequencing through the lower bands 4 through 1 so as to produce a smoothtransition back to the uncontrolled mode.

Assuming the vehicle enters band 5 through program block 318 and that apositive indication is reached in the next program block 286 the brakeforce is periodically incremented upwardly by reducing the brakepressure as indicated in program block 294 by the increment X(psi) forevery timing cycle until the slip value qualifies the program for entryinto another band. This is indicated in FIG. 5 where the first step fromthe hold off pressure of 400 psi represents an abrupt drop to 200 psiand three additional incremental drops of approximately 33 psi so as toincrease the parking brake force through spring action with eachincrement.

After the first large pressure drop and the three incremental pressuredrops the slip condition in the example represented by FIG. 5 is shownto enter band 4; i.e., the slip has been reduced to the point where thevalue is between 1.5 and 1.7. In this condition program blocks 320 and322 cause the brake force to be increased by a smaller increment Y (psi)for every timing cycle. Accordingly, the system approaches the fullbrake force condition in a gradual curve where the increments becomesmaller toward the full brake force condition.

As is also represented in FIG. 5, reaching the lower slip values so asto qualify for successive entry into bands 3, 2 and 1, causes graduallyincreasing incremental reductions in brake force until the system isback to the uncontrolled condition represented by the full brakepressure condition of 400 psi. To this end band 3 is the mirror image ofband 4 and causes incremental reductions in brake force throughincremental increases in brake pressure; band 2 is the mirror image ofband 5 in causing large (X) incremental reductions in brake force. Band1 has no counterpart and causes fairly large reductions in brake forceas represented by the increment W in the example diagram of FIG. 5.

From the foregoing it is apparent that the slip control system operatesto detect slip, to apply a braking force to the slipping wheel and toperiodically and incrementally modulate the brake force eitherpositively or negatively in accordance with the degree of slip which isdetected by the system. The use of a drive shaft speed signal provides afailsafe condition wherein a false zero speed indication from one of thewheel speed transducers is immediately recognized and the slip controlsystem appropriately disabled. Vehicle turning conditions giving rise toa slip indication of 1.5 or less but with both wheels turning, do notcause entry into the slip control modes since the conditions necessaryfor band 5 entry are not satisfied. On the other hand, once the slipcontrol mode has been entered via band 5, a new slip threshold isestablished such that the system can exit from the slip control modethrough at least three incremental steps represented by bands 3, 2, and1.

It will be appreciated by those skilled in the art that is it notessential to incorporate all of the steps represented in the flow chartof FIG. 4 in a given system, nor is it necessary to utilize amicroprocessor. However, such an implementation is deemed to be the bestmode of implementing the invention due to the broad and widespreadcommercial availability of suitable microprocessor circuits, thewidespread understanding of programming techniques for suchmicroprocessors, the cost reduction in such integrated circuitry whichhas been realized in recent years, and the flexibility which aprogrammable device affords.

Other aspects, objects and advantages of this invention can be obtainedfrom a study of the drawings, and disclosure and the appended claims.

We claim:
 1. In a system for balancing the power transfer between twovehicle wheels (200,202) which are driven by a common input shaft (204)through a differential unit (206) when one of said wheels losestraction, said system being of the type which includes means (216,222)for generating wheel speed signals, control means (220) for developing aslip signal related to the difference between said wheel speed signals,and means (212,214) operated by said control means (220) for applying abraking force to the wheel which loses traction, the improvementcomprising:means (226) for generating a signal representing input shaftspeed, said control means (220) including means (242) for comparingwheel and input shaft speeds and preventing actuation of the brakingmeans (212,214) for slip control purposes until the ratio of the speedof the wheel which loses traction to the speed of the input shaftexceeds a predetermined value.
 2. Apparatus as defined in claim 1,wherein the control means (220) further comprisesmeans (244) fordefining a plurality of slip signal value bands each having defined slipvalue limits, and means (230, 232, 234, 256) for incrementally changingthe braking force in accordance with the band within which the currentslip value falls.
 3. Apparatus as defined in claim 2 wherein thedefining means (244) further defines both positive and negativeincremental changes, each change being associated with a given slipvalue band and the negative brake force incremental changes beingassociated with lower slip value bands.
 4. Apparatus as defined in claim2 wherein the means (230, 232, 234, 256) comprises a source (234) ofbrake fluid pressure and signal responsive means (232) connected to saidmeans (242) and responsive to outputs therefrom for varying the brakefluid pressure applied to said wheels.
 5. Apparatus as defined in claim4 wherein the means (230, 232, 234, 256) further comprises wheelselection means (230) responsive to outputs from said means (242) tovary the brake fluid pressure to only one of said wheels.
 6. Apparatusas defined in claim 2 wherein said means (242) comprises timer means(246) for cyclically recalculating the slip signal value.
 7. Apparatusas defined in claim 2 wherein the means (230, 232, 234, 256) furthercomprises a duty cycle generator (256) connected to receive outputs fromthe control means (220), and means (260 and 262) responsive to theoutput of the duty cycle generator for varying the brake fluid pressureapplied to the other of said wheels under slip conditions.
 8. Apparatusas defined in claim 1 wherein the predetermined value is substantially1.5.
 9. A method for balancing the application of power to two vehiclewheels (200,202) which are driven by a common input shaft (204) througha differential unit (206) when one of said wheels loses traction and forscreening out false indications of traction loss comprising the stepsof:developing first and second signals representing the speeds of thedifferentially driven wheels; developing a third signal representingslip as a function of the difference between the first and secondsignals; developing a fourth signal representing input shaft speed;comparing the input shaft speed signal to the larger of the wheel speedsignals; and applying a brake force to the higher speed wheel only whenthe ratio of the wheel to shaft speeds exceeds a predetermined value.10. A method as defined in claim 9 wherein the value is at least 1.5.11. A method of preventing spurious operation of a selected-brake typewheel slip control system due to wheel speed transducer failurecomprising the steps of:determining the presence of a slip condition bycomparing wheel speed transducer signals from two differentially drivenwheels, comparing the speed of the faster rotating wheel to the speed ofa differential input element such as a drive shaft; and acting tocontrol slip through selective brake application only if the ratio ofthe faster wheel speed to the differential input speed exceeds apredetermined value.